Transcript Perspective in Object-Oriented Programming

Perspective in
Object-Oriented Programming
How to be an Informed Consumer
of Software Theory and Practice
Optimization
A component of Programming
Not just the algorithms, also
Readability
Maintainability
Testability
Etc.
“Why callest thou me
good?”
What does “good” mean?
What does quality mean?
Long discussion: Zen, and the
art of Motorcycle Maintenance
My view: Quality is the measure
to which something fulfills its
purpose
Purpose is individual, can be
subjective
Good Programming
What makes a program “good”?
Speed?
Stability?
Maintainability?
For any non-trivial criteria,
maximizing “goodness” is hard
Optimizing for one criteria may
reduce performance of another
On Being an Informed
Consumer
Computer Scientists are consumers
At a minimum, they require a
programming paradigm and
compliant language
More often, they also require
Methods, concepts, philosophies
Tools such as IDEs, debuggers
Tools such as UML, patterns
Motivations of the CS
Producers
Money (pure and simple)
Ego
Fame and/or Prestige
Your best interest (rarely)
NOTE: The tools you use
benefited one or more persons
How to Choose Wisely
You do need to buy bullets
How do you choose wisely?
They’re obviously not silver…
Do they have any value?
No-Free-Lunch Theorem
“[…] all algorithms that search
for an extremum of a cost
function perform exactly the
same, when averaged over all
possible cost functions” -
(Wolpert & Macready, 1995)
NFLT Restated
Optimizations, when averaged
over all possible problems,
perform exactly the same.
Applied to CS: programming
tools/methods/etc., when
applied to all possible
programming problems,
perform exactly the same.
NFLT Restated Again
“NO
SILVER BULLET”
--Brooks
Context: Key to
Optimization
Optimization algorithms
Perform well on function classes
Class size not impacted by NFLT
Identification of class is hard
Back to Programming
Tools/Methods perform well on
software problem classes
Determining the class is hard
Reverse engineer tool/method for
approximation of the class
Also requires understanding of
solution space, and “goodness”
function (don’t optimize wrong
thing)
One View of Software
Continuous range of abstraction
Requirements
Architectural/Conceptual
Design
Algorithmic
(Programming) Linguistic
Etc
Abstraction Interfaces
As humans, we discretize
Create abstraction layers
Create interfaces between
layers
Working in Layers
Most abstraction layers can
influence above or below
However, flow is from abstract
to concrete
Most opposite flow is some
form of feedback
Common Design Error
Design with insufficient input
from higher abstraction
Feedback alters view of higher
abstractions
Adventure Example: lighted
room
Optimization Problem
The reverse abstraction flow
problem is picking the wrong
optimization
This error was independent of
programming paradigm,
language, and tool
OOA and OOD
Personal Opinion:
Object-oriented programming is
generally less effective without
object-oriented design
Object-oriented design is
generally less effective without
object-oriented analysis
OOA Mini How-To
(For more: read a book)
Organize problem space into
objects
Objects may or may not have ANY
connection with implementation
objects later
Objects have:
Structure
Behavior
State
OOA and UML
You CAN use UML, BUT
DO NOT be tempted to translate
to code
You don’t have to ascribe
properties and methods to UML
objects. For OOA, you generally
don’t
OOA: Structural
What
What
What
What
What
is it?
is it composed of?
properties does it have?
is it related to?
does it interact with?
OOA: Behavioral
Start with the answer to “What
does it interact with” question
What kind of messages does it
send?
What kind of messages does it
receive?
What kind of messages does it
require?
OOA: State
Describe basic state machine
of major objects
Tie in with messages from
OOA: Behavioral
Why so much structure?
OOA lends itself to planning
more structure than behavior
or state
Why?
OO[A/D/P] holds implicit belief
that form follows function.
Example: Adventure
What is a Game?
This is a significant question
Does a Game have rooms?
Does a Game have a World that
has rooms?
Does a Game have input and
output?
What are the relationships?
Concrete Example:
Rooms
Does a room have a light
property?
If so, what properties does a
flashlight have?
Let’s do this on the board
One Concluding Note
Any work in OOA, OOD, or
OOP should be driven by the
requirements
This is the most broken
commandment in computer
science
On To Patterns
What are they?
According to Steve Bilow:
“… patterns have the potential to
permanently alter the software
engineering field, catapulting it
into the realm of true elegant
design.”
Yet More Praise for
Patterns
The worlds of Tom Cargill
“After a modest investment of time
with it, most C++ programmers
will be able to start applying its
‘patterns’ to produce better
software”
Hurray! The Silver Bullet!
I guess my lecture is all wrong
We’ve finally found the universal
optimizer
Also, it seems like it makes
everything about software better
Don’t you feel all warm and
fuzzy?
But… wait…
Design Patterns, the book by
the gang-of-four came out in
1995
I’ve worked for four companies
since that time
Many of us trained in patterns,
still struggle to get designs
right…
That’s odd…
Because the introduction to
Design Patterns says this:
“Put simply, design patterns help a
designer get a design ’right’
faster” (page 2)
Where is the Disconnect?
Personal opinion: forgetting
that the bullet is not silver
Design patterns are an
amazingly effective tool
In fact, they are an optimizer
They optimize a class of
problems
So, let’s backtrack in time…
A History of Patterns
The concept of patterns was
first formalized for architecture
by Christopher Alexander
Three most cited books:
The Timeless Way of Building
A Pattern Language
The Oregon Experiment
History … Again
TWOB and a Pattern Language
should be required reading
In class:
What is a pattern? (PL, p. x).
What is a pattern language?
(TWOB, p. 202)
Advice: check you library
What is a Pattern,
again?
We’ve got three types of
patterns going on here:
“pattern” - see dictionary.com
Pattern - as per Alexander
PatternTM - as per GoF
We need to talk about all
three.
“pattern”
Dictionary.com:
“A model or original used as an
archetype”
There are patterns in every
aspect of software engineering
at every level of abstraction
In reality, our world view is
based on patterns
Pattern
Describes an architectural problem
that occurs over and over again
Proposes a core solution that can
be done a million ways without
doing it the same way twice
Part of a larger Pattern Language
PatternTM
Descriptions of communicating
objects and classes that are
customized to solve a general
design problem in a particular
context
Names, abstracts, and
identifies the key aspects of a
common design structure
Getting to the Core of
TM
Patterns and Patterns
The common idea is: don’t reinvent the wheel.
What separates these from
“patterns” is that they have a
name, some analysis, and a
written form
Limitations on Applying
Alexander
Alexander is discussing
intimate architecture
In other words, the producer is
the consumer of the design
Slow conextual changes
Architecture has thousands of
years of experience
Limitations on the Gang
of Four
Narrow view of a PatternTM
Rigidity in pattern maintenance
Worst of all:
A significant dearth of work on
when to use and when not to
use a pattern
TM
Pattern
Limitations on
Use in Software
Superficial similarities between
past and present problems
Reverse abstraction flow
Temptation to apply to wrong
level of abstraction
Fragility to context changes
(capture only; no-meta)
NO LANGUAGE
TM
Patterns
Using
“Tastefully”
Analysis of context
Keep to the requirements
Modify for contextual changes
Use to solve problems that arise in
design, not as “cookie cutters” in
the design space
FINALLY: it is about resolving
forces!!!!
Using a Pattern
Language
Alexander believed architecture
was dead because of a dearth of
communication
Expand view beyond PatternsTM to
something more like Alexander’s
Pattern Language
Don’t be too proud to learn from
somebody else’s “patterns”
Recognizing “patterns”
and Pattern-like things
Requirements patterns
“Best Practices” is Pattern-like
“Effective Java” is Pattern-like
Final Example
Multi-paradigm Programming
Significant context change
Don’t wait for new patterns to
be captured
Explore the meta-space to
translate to the new context
My Final Thought about
the Patterns Community
One significant member said:
Most people aren’t qualified to
talk about what patterns are
Only he, and a few others, with
time and means should venture
into that space
Everyone else should just
content themselves to
write/mine patterns
Summary
YOU are a consumer
Be informed, understand
motives
Remember NFLT
Understand the theory and
background of what’s being
sold to you
Never trust marketing